“It’s very important that the British fishing community is protected in Brexit.

“They were very badly let down when we joined in 1973, and they shouldn’t be used as an easy negotiating offer in this round.”

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“It’s very important that the British fishing community is protected in Brexit. They were very badly let down when we joined in 1973, and they shouldn’t be used as an easy negotiating offer in this round."

The intervention by the chairman of the powerful Tory European Reform Group threaten a new Tory truce on Brexit.

It follows a Cabinet row over the totemic Brexit issue between Chancellor Philip Hammond and Environment Michael Gove last year.

Mr Rees-Mogg also flagged up potential alarm about the PM’s pledge to sign up to a series of EU watchdogs, such as on medicines, saying he also had concerns about “the details surrounding how we will subscribe to their agencies”.

Mr Rees-Mogg added: “The way the Prime Minister set it out sounded perfectly reasonable, but the devil will be in the detail as so often.”

Campaign group Migration Watch UK also warned Mrs May last night not to water down border controls, newly won by Brexit.

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British fishing boats, at work in the north sea

Downing Street admitted some elements of the nation’s new immigration system would be thrashed out in talks with the EU.

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“One possibility, though unlikely, is that we reach an agreement on implementation then fail to reach one on a long-term agreement,” he told MPs yesterday.

He added that billions could be set aside for “contingency” planning.

Challenged whether the transition talks could continue into 2021, Mr Hammond would only say the UK has “no plans to make provision for an extension”.

Speaking to the cross-party European Scrutiny Committee, he said a transition deal was vital for businesses both in the UK and EU.

He warned airlines may have to rip up schedules for next summer unless a transition deal was thrashed out by the end of the month.

He said: “The implementation period is very much in our view in the interests of both sides. It will create certainty to allow business to plan and deal with immediately pressing issues.

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Traditional British fishing vessels, Beached in Cornwall.

“For instance airlines will need to know on April 1 whether they can safely schedule flights in April 2019.

“There are lots of practical issues that are going to be very problematic across the Continent of Europe unless we agree this implementation deal. I believe it will happen as in everyone’s interest it does happen.”

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